[K]

It's counter-intuitive that Water magic is not that great fighting underwater, but it's true.

What you need is Nature magic and Air magic for battle magic, and death and/or Earth for troops.

Battle magic for underwater includes Lightning Bolts and Poison Clouds, and a few army boosting spells like Fog Warriors and Army of Lead, for example.

For troops, building water-breathing items is a bit of a waste when you can just summon Marble Warriors or undead.

Since you are fighting AI, SCs are the best. Kit out a few Banelords or Elemental Royalty and take it to them.